1. Field of the Invention
The present invention relates to a flash charging circuit and a method of driving the flash charging circuit. More particularly, the present invention relates to a flash charging circuit wherein a main capacitor is charged effectively without any waste of power.
2. Description of the Related Art
To obtain a high voltage necessary for a discharge tube to flash, a main capacitor is charged up to the high voltage in a flash charging circuit, prior to each flashing. For quick charging of the main capacitor, a low voltage of a power source, such as a battery, is transformed to a higher voltage.
In a conventional flash charging circuit, a drive pulse signal is inputted at a regular interval in a single semiconductor switching element such as a transistor or a field effect transistor (FET), to turn the switching element ON and OFF, so as to intermittently apply a primary voltage through a primary coil of a transformer. In this type of flash charging circuit, a primary current flows through the primary coil while the semiconductor switching element is ON, and the primary current induces a secondary voltage in a secondary coil of the transformer. A secondary current caused by the secondary voltage is half-wave rectified, to be supplied to a main capacitor.
Because the secondary voltage is induced only while the semiconductor switching element is ON, the main capacitor is charged in an intermittent fashion. Accordingly, the total charging time necessary for charging the main capacitor up to the predetermined voltage is certainly long. To solve this problem, a flash charging circuit is disclosed in Japanese Laid-open Patent Application No. 7-29689.
In the flash charging circuit of this Japanese publication, a pair of semiconductor switching elements are connected to opposite terminals of a primary coil of a transformer, and a battery is connected to a center tap provided in a middle portion of the primary coil, such that a primary current flows from the center tap to the first terminal while one of the switching elements is driven, and that a primary current of an inverted phase flows from the center tap to the second terminal while the other switching element is driven.
First and second switching signals or drive pulse signals having inverted phase are applied to the switching elements to drive them alternately with each other. Thereby, the currents flow through the primary coil alternately in the opposite directions, so that an alternating secondary current is induced in a secondary coil of the transformer. This operation is called push-pull operation. The secondary current is full-wave rectified by a rectifier circuit, such as a diode bridge circuit, and is used to charge a main capacitor. Because one of the switching elements is ON while the other switching element is OFF, the secondary voltage is continuously induced, so that the main capacitor continues being charged. Thus, the improved flash charging circuit shortens the total charging time necessary for charging the main capacitor up to the predetermined voltage.
On the other hand, it is known in the art that a semiconductor switching element needs a certain response time from the application of a drive signal to the start of current conduction, as well as from the termination of the drive signal to the end of current flow. It is also known that a falling response time of a semiconductor from the end of driving to the actual end of current flow is generally longer than a rising response time of the same semiconductor from the start of driving to the actual current conduction, although both response times vary according to the type of the semiconductor.
Accordingly, in the latter flash charging circuit, since the switching signals have inverted phases, a drive pulse starts to be applied to the second switching element simultaneously with the trailing edge of the latest drive pulse that has been applied to the first switching element, or vise versa, so that the second switching element is turned ON before the first switching element is completely turned OFF, or vise versa.
Hereinafter, a time period from the start to the end of current conduction of each semiconductor will be referred to as ON-period, and a time period when the semiconductor does not conduct any current will be referred to as OFF-period. Then, it can be described that the ON-periods of the first and second switching elements overlap with each other in the conventional flash charging circuit using the inverted phase switching signals. While both of the switching elements are ON, the primary currents of the opposite directions concurrently flow through the primary coil of the transformer. As a result, magnetic fields of opposite directions are induced concurrently in the primary coil, so that they cancel each other. Consequently, the primary voltage is not induced in the secondary coil, so that the main capacitor is not charged. However, the power of the battery is still consumed at that time because the primary currents are flowing through the primary coil. That is a waste of electric power.